This invention relates to blue pigmented phosphors for color cathode ray tubes.
In recent years so-called pigmented phosphors in which a pigment which reflects the emission color of the phosphor and absorbs other visible light is bonded to the surface of the phosphor have been used to improve screen contrast in cathode ray tubes used for color televisions.
The pigments used at present are cobalt aluminate and ultramarine blue for blue, and iron oxide, cadmium sulphoselenide and indium sulphide for red, and in all cases they are controlled to an average grain diameter of 0.05 .mu.m-5 .mu.m and bonded to a phosphor of 0.1 to 21 .mu.m. Apart from being matched to the emission spectrum of the phosphor, the pigments used in the phosphors for these cathode ray tubes also have excellent emission characteristics. It can be assumed that the better the tinting strength of the pigment the better the pigment. However, in practice the pigments chiefly used in color cathode ray tubes are cobalt blue for blue and iron oxide for red. Ultramarine blue and cadmium sulphoselenide are pigments with better spectral reflection characteristics than these but are not used at present. The reason for this, apart from the fact that there is a problem of pollution with cadmium sulphoselenide, is the fact that pigments such as ultramarine blue fade during the manufacturing process of the color cathode ray tube.
There are two kinds of fading, namely the oxidation fading which occurs when the sodium polysulphide in ultramarine blue is oxidized by the oxygen in the air and by the heat in the color cathode ray tube manufacturing process (hereinafter referred to as air oxidation), and oxidation fading caused by chromates such as ammonium dichromate and sodium dichromate which are used as sensitizers when the phosphor membrane is formed (hereinafter referred to as chromate oxidation).
Air oxidation occurs particularly during the cathode ray tube manufacturing process. This is partly due to the fact that the temperature is raised to about 420.degree. C. when the section which forms the screen to which the phosphor is applied (face) is sealed to the anode and electron gun section (funnel and neck) using frit glass as the sealant. It is also partly due to the fact that the temperature is raised to close to 450.degree. C. to achieve thermal decomposition of the polyvinyl alcohol used when the phosphor membrane is formed and of the organic films used to level the aluminium membrane after formation of the phosphor membrane. The limit of stability of ultramarine blue itself is 300.degree. C. and if it is heated above this temperature the ultramarine blue is oxidized in accordance with the following equation: ##STR1##
In other words, when the temperature exceeds 300 C. the sodium polysulphide in the ultramarine blue is oxidized by the oxygen in the air to sodium sulphate, the characteristic blue coloration of ultramarine blue disappears and fades to white.
The chromates (sodium dichromate and ammonium dichromate) which are used as sensitizers during the formation of the phosphor membrane are the cause of the chromate oxidation. These chromates have a powerful oxidizing action, as is evident from the fact that they are generally used as anti-reduction powders and as oxidizing agents for metals.
When the phosphor membrane is formed a slurry is prepared by blending together the phosphor, polyvinyl alcohol, dichromates and dispersing agent in the form of polyhydric alcohol. This is applied to the inner surface of the face which will form the screen of the cathode ray tube, and dots or stripes of the phosphor are formed by drying, exposure and development. During the exposure the polyvinyl alcohol promotes photopolymerization by the light absorbed by the chromate, and the chromium changes from Cr.sup.+6 to Cr.sup.+3 and loses its oxidizing power. In other words, it forms a compound which does not influence the fading of ultramarine blue, but the presence of unreacted 6-valent chromium remains a problem with respect to fading of ultramarine blue. This unreacted 6-valent chromium is removed to some extent by showering in the subsequent development process but the 6-valent chromium which has not been removed by these processes fades the ultramarine blue from blue to white in the baking process of the cathode ray tube process according to the following equation: ##STR2##
This case is similar to the fading of the ultramarine blue due to air oxidation, in that the sodium polysulphides in the ultramarine blue fade forming sodium sulphate by oxidation. This is because the 6-valent chromic oxide, which is powerful oxidizing agent, releases oxygen at a temperature of 275.degree. C. with the formation of 3-valent chromium, and this oxygen oxidizes the sodium polysulphide to sodium sulphate. This extent of fading due to chromate oxidation of the ultramarine blue is even more severe with air oxidation. However, neither of them can be disregarded when pigmented phosphors are used and for this reason it has until now been extremely difficult to use ultramarine blue as the blue pigment. Japanese Patent Disclosure No. 60-94491 has a method for improving this type of ultramarine blue fading. This is a method where the surface of the pigment is coated with a silicon compound and, as shown in the embodiment, the coating is an inorganic silicon compound.
There is also another method shown in Japanese Patent Publication No. 59-10709 in which the surfaces of the pigment and the phosphor are covered with a continuous film of silica. However, the film obtained in this method is made of inorganic material. If the above methods are used the fading of ultramarine blue in the baking process of the color cathode ray tube can be prevented to a certain extent, but it is not sufficient and, especially with the slurry method which uses dichromate, there is considerable fading.